Known in a prior art is the method for preparing naphthenic acids by oxidizing a petroleum fraction boiling out in a temperature range of 250.degree.-500.degree. C. with oxygen in the presence of a catalyst, a finely dispersed potassium permanganate (USSR Inventor's Certificate No. 137,515). According to this method, oxidized is the fraction comprising the following components, in percent by weight:
______________________________________ naphthene hydrocarbons 79 paraffin hydrocarbons 12 aromatic hydrocarbons 3 ______________________________________
The acid value of said fraction is 32.2 mg KOH per gram of the fraction; its viscosity at a temperature of 50.degree. C. is 59.4 centistokes. The petroleum fraction is oxidized at a temperature of 150.degree. C. at an oxygen consumption of 0.25 cu.m per hour. The reaction continues for 32 hours; the catalyst consumption is 0.2 percent of the taken raw material.
The oxidation gives a complicated mixture of organic oxygen-containing compounds, among which there are acids of the aromatic, naphthenic and fatty series, the products of their peroxidation, viz., oxy acids, ether acids, keto acids, lactones and lactides.
The obtained mixture of oxygen-containing compounds is treated with a 20 percent aqueous solution of alkali. This treatment gives a mixture of alkaline salts of organic oxygen-containing compounds and unsaponifiable organic oxygen-containing compounds. Unsaponifiable oxygen-containing compounds are separated from the obtained mixture, for example, by extraction with organic solvents. The remaining mixture of the alkaline salts of organic oxygen-containing compounds is treated with a 30 percent sulphuric acid to separate a mixture of oxygen-containing organic compounds which include mono-, di- and poly-basic acids of the aromatic, naphthenic, and paraffin series; the mixture contains also non-oxidized materials.
The main disadvantage of this method is that the finely dispersed catalyst used in this process is not selective toward the process of preparing naphthenic acids. Moreover, another disadvantage of this method is that naphthenic acids are a component of a complicated mixture of oxygen-containing organic compounds, while high-quality naphthenic acids cannot be obtained by this process.